1. Field of the Invention
The invention relates to a method for forming bodies of foamed plastisol resin, and more particularly to an improved method for forming bodies of foamed plastisol within closed molds.
2. Description of the Prior Art
The prior art, of course, is replete with various formulations for plastisol as well as various formulations for chemically blown plastisols.
A typical formulation, in parts by weight, for chemically blown plastisol is as follows:
______________________________________ CHEMICALLY BLOWN FOAMS (Basically Closed Cell Structure) Filled Filled Soft Soft Hard Hard Foam Foam Foam Foam ______________________________________ PVC emulsion plastisol resin homopolymer.sup.a 70 70 70 70 Extender resin.sup.b 30 30 30 30 DOP 20 80 50 50 BBP 70 10 10 10 Epoxy plasticizer 5 5 5 5 Blowing paste azodicarbonamide 6 6 6 5 Zinc blowing catalyst 3 -- -- -- Dibasic lead phosphite -- -- 6 5 Ba-Cd-Zn stabilizer -- 3 -- -- CaCO.sub.3 -- 20-25 -- 20-25 Surfactant 1.5 -- -- 2 Approximate foam density lb/cu ft 19 21.5 20 22.5 Blowing temperature 360.degree. F. 360.degree. F. 360.degree. F. 365.degree. F. ______________________________________ .sup.a Diamond PVC-7602, inherent viscosity 0.90. .sup.b Diamond PVC 7-44, inherent viscosity 0.83.
One of the most commonly used chemical blowing agents is azodicarbonamide, which tends to produce foam with a high closed-cell content. Through a proper selection of a heat stabilizer-activator, the temperature at which cellular expansion will occur can be adjusted with suitable accuracy. Hence, a formulation can be chosen which will yield the desired cellular structure and foam density. For example, given a plastisol having a fusion temperature of 320.degree. F. a blow agent which decomposes at approximately 360.degree. F. produces a blown plastisol having a foam density suitable for many practical uses. Since formulations and temperatures are variable, and can be established emperically, a more detailed discussion of the formulations for plastisols is omitted in the interest of brevity.
One technique for forming bodies of foamed plastisol is more fully described in U.S. Pat. No. 3,904,720 to Gerald D. Sjostrand. This method includes the steps of dipping a heated mandrel in a liquid bath of plastisols, including a blowing agent, for thus forming a gelled body on the surface of the mandrel, and thereafter immersing the mandrel in a liquid bath maintained at a temperature at which the blowing agent decomposes. While this method serves quite satisfactorily for its intended purpose, it is not readily adaptable for use in producing bodies requiring a closed mold in the production thereof.
It has been suggested that foamed articles can be produced employing a closed mold cavity. For example, U.S. Pat. No. 3,432,581, to Jacob J. Rosen, discloses a method for producing foamed articles from foamable plastic compositions, specifically a foamable polyvinyl chloride plastisol, wherein the final product is formed of a continuous foam structure, including a cellular inner portion and a substantially smooth outer skin. The preferred technique therein disclosed includes the steps of mixing and aerating a plastic composition, distributing a gelled layer of plastic composition on the surface of the mold cavity by rotating the mold, while heating, and subsequently fusing and foaming the plastisol.
It is important to note that the patent discloses a technique wherein a flowable plastisol is placed in a mold cavity, the mold cavity closed, with the mold haves being joined together, and thereafter the mold is rotated about a plurality of axes, while heating, in order to distribute the plastisol throughout the mold cavity, to form a gel layer on the cavity walls.
This technique suffers from several obvious disadvantages and cannot readily be employed by operators lacking suitable equipment. For example, the filled and closed mold must be rotated in various directions, while being heated, in order to assure that a layer of gelled plastisol is formed on the internal surface of the mold cavity, before foaming is initiated.
Moreover, as should be apparent, the heating of a mold at its external surfaces causes plastisol contained therein to gel and then foam inwardly. Inward foaming tends to cause a layer of foamed material to be established about the inner core of the mold cavity. This thin layer acts as a thermal barrier and prevents the cavity from being uniformly heated. This tends to result in a product of a non-uniform density. Consequently, the resulting product lacks uniformity in size and configuration.
Finally, when employing this technique for forming bodies of foamed plastisol, the flowable plastisol, under the influence of gravity, tends to seek its lowest level so that the position of the mold, prior to gelling, becomes critical.
Consequently, it should be apparent that a need exists for a practical, and improved method for forming bodies of foamed plastisol utilizing a closed mold cavity.
Accordingly, it is the purpose of the instant invention to provide a simple, practical, and economic method for forming bodies of foamed plastisol resin utilizing a closed mold cavity, whereby the difficulties and disadvantages of attending the methods as taught by the prior art are avoided.